Optical systems are systems that employ optical lenses, mirrors, sensors, e.g., infrared sensors and the like, etc., to monitor a specified parameter. To optimize the performance of optical systems, it is desirable that the optics within the system be free from contaminants that can interfere with the lens's, sensor's, etc., performance. Contaminants, as used herein, include airborne molecular contamination (AMC), which also is referred to as a Collectable Volatile Condensable Material (CVCM), e.g., a soil. As is known in the art, AMCs cover a wide range of contaminants present in the air and can lead to contamination in the form of chemical films, sometimes as thin as a single molecule.
AMCs can cause yield losses due to changes in the chemical, electrical, optical, and physical properties of product surfaces. As contaminants accumulate on optical surfaces, the contaminants physically absorb and scatter incoming light, thus distorting the quality of the spherical wavefront. When the information contained in the spherical wavefront is distorted, the resulting image also is malformed and the overall performance of the optical system is degraded.
Every material introduced into a system is a potential source of AMCs. The chemical composition of a material, its surface area and its use temperature ultimately determine the levels of contamination introduced into the system. The contaminants can be introduced through numerous means, the most common of which being through contaminants on subcomponents, contaminants introduced during assembly of the optical system, e.g., assembly of the subcomponents, and contaminants introduced from outgassing of materials within the subcomponents.
As is known in the art, outgassing is the evolution of embedded substance, with a non-zero vapor pressure from a material over time. Outgassing occurs when a material is placed in a low pressure environment and can be accelerated under conditions of elevated temperature. As a material undergoes outgassing, some of its constituents are volatilized and the material experiences a weight loss, measured as percent total weight loss, and a certain percentage of the volatile constituents are condensable upon nearby surfaces. This second property is the more critical, as the condensable matter may contaminate sensitive optical or thermal control surfaces.
In designing and manufacturing optical systems, it is desirable to know the end life (e.g., worst case) contamination that can be introduced into the system. With such information in hand, the effects of design and manufacturing changes on the performance of a device can readily be ascertained without the need to physically construct and test the device.
Estimating the effect of outgassing products on the end of life performance of optical systems requires knowledge of the absorption coefficient of the aggregate outgassing soil as a function of wavelength associated with an aggregate film thickness. An aggregate film, as used herein, is a contaminant film that is formed from the combination of all soils outgassed within a system. One approach to solving this problem is to estimate the contribution of each individual material present to the aggregate soil. This approach requires that a spectrum, such as an infrared spectrum, be acquired from a sample of outgassing soil from each individual material, where the thickness of the corresponding soil sample is known. Unfortunately, the thickness of the soil sample usually is not known, and thus estimating the contribution of each individual material present to the aggregate soil is not feasible.
Conventional methods have attempted to estimate the thickness of each individual film. Unfortunately these methods have proven to be ineffective for molecular contaminant films that exceed 10 nanometers in thickness. Additionally, conventional methods do not consider the chemical composition of the total aggregate molecular contaminant film. As a result, such methods are of limited use, since accuracy falls off sharply as variations in chemical composition of the contaminant film increase.
Accordingly, there is a need the art for a system and method that accurately models the spectral character of a molecular contaminant film based on the components that generate the film. Additionally, it would be advantageous for such a system and method to consider the chemical composition of the aggregate of the molecular contaminant film in modeling the molecular contaminant film.